The two sentences that best summarize the excerpt from Nathaniel Hawthorne's short story "Dr. Heidegger's Experiment" are:
1) "Are we grown old again, so soon?" cried they, dolefully.
This sentence perfectly summarizes that the people that drank from the Fountain of Youth finally realized that the effect of its waters is <em>temporary</em> and not permanent.
2) "Well, I bemoan it not; for if the fountain gushed at my very doorstep, I would not stoop to bathe my lips in it; no, though its delirium were for years instead of moments."
This second sentence summarizes not only the excerpt, but most of the short story as well. It states that Dr. Heidegger just wanted to see what happened when the waters were drank, and that the effect is, indeed, only a fantasy: the four "friends" were never truly young again, it was only an illusion, a <em>delirium</em>.